Heat exchangers of the above type are particularly suitable for operating with gases at high gas temperatures and under rapidly changing thermal conditions. Heretofore, these heat exchanger have been made by rigidly connecting the heat exchange tubes to the manifold ducts by soldering or welding. It is also known to form the ducts of at least two half-shells which are assembled to each other. Alternatively, the ducts can also consist of individual shorter duct sections which are arranged one after the other and soldered together.
Rigid attachment of the parts by soldering or welding has heretofore been considered necessary in order to prevent leakage, during operation, between the heat-exchanging fluids. This is a real problem due to application of considerable thermal stresses, particularly during nonstationary operation and because of external vibrations and vibrations caused by the flow of gas.
This construction has the disadvantage that, in the case of leaks, which can occur either due to defective manufacture or by fatigue of the material, expensive repairs or even replacement of the entire heat exchanger is frequently necessary. Upon application of impact loads transverse to the axis of the manifold duct, high stress peaks can occur at the points of connection between the ducts and the heat exchange tubes due to transient forces, resulting in the danger of cracks and therefore leaks since the ducts are connected to a large number of heat exchange tubes. As a consequence of cracks, there is a local weakening of the rigidity and strength of the ducts, as a result of which there is brought about a progressive increase in the local stress peaks and thus progressive damage until breakage occurs
This is particularly critical in the case of application of impact forces on the ducts in the direction of the axes of the heat exchange tubes, since the region of the periphery of the duct at which the U-shaped tubes are connected is the region of the highest tensile and compressive stresses. In this case, therefore, due to the low moment of resistance of the ducts, rapid progressive damage takes place upon alternating or impact loading. The supporting effect produced by bringing the heat exchange tubes together on the one side upon bending of the duct in the direction of the stress plays a minor role here.